In the fracturing of subterranean formations for the production of hydrocarbons, it is common in the art to develop fractures in the zone of interest by pumping a fluid at relatively high pressures which overcome the pressures of the over-burden on the rock in the zone of interest to create and extend fractures in the rock. Such fractures become channels for return of the desired hydrocarbon fluids to the wellbore. Nearly any fluid, given enough pressure, can be used for the fracturing process. However, fracturing fluids generally have a second function, namely that of transporting a particulate proppant material into the fractures so that, upon release of the fluid pressure, the proppant remains within the fractures to hold them open. The rheology of the fluid also acts to influence the extent of the fractures away from the wellbore.
In order to affect the rheology and increase the proppant carrying ability of a fracturing fluid, organic polysaccharide materials such as cellulosics and/or natural and synthetic gums are added to the fluid to increase its viscosity and proppant transport capability. Additionally, particularly with the use of gums and modified gums such as guar, hydroxypropyl guar and the like, the viscosity and proppant transport capability can be even further increased by the use of crosslinking additives. Some well-known crosslink additives include borates as described in U.S. Pat. No. 3,974,077 and titanate or zirconate organometallic crosslinking agents such as described in U.S. Pat. Nos. 4,757,080 and 4,686,052, respectively.
Each of these crosslinked fracturing fluids has its own particular advantages. Borate crosslink systems have the advantage of being less damaging to fracture conductivity because the gels can be broken more efficiently and removed from the fractures leaving fracture passages free for the production of hydrocarbon fluids. Because of thermal breakdown, however, borate crosslinked fracturing fluids can only be used in relatively low temperature (50-225.degree. F.) formations. Titanium and Zirconium organometallic crosslinked fracturing fluids have the advantage of being relatively resistant to thermal breakdown in high temperature (greater than 200.degree. F.) formation environments. However, it is well-known that such high temperature stable, organometallic crosslinked fracturing fluids are somewhat more damaging to proppant conductivity within the resultant fracture. Incomplete breaking of the crosslinked gel can result in relatively large amounts of gel residues remaining in the fracture passages thereby lowering fracture conductivity.